Although hepatitis C virus (HCV) is a leading cause of morbidity and mortality worldwide, the effects of viral gene expression on infected cells remain unclear in vivo. Previously, we reported the construction of transgenic mice expressing HCV structural proteins (core, E1 and E2) and showed that expression of HCV structural proteins is not directly cytopathic in this animal model. Our laboratory has now generated transgenic mice expressing HCV full-length polyprotein and is developing a system for the inducible expression of HCV transgene. Based on the tetracycline regulatory system, we established a binary transgenic model in which the conditional expression of two transgenes, SV40 T antigen (TAg) and LacZ, can be tightly regulated in the liver by administration of tetracycline. Mice with tTA and TAg transgenes developed hepatocellular adenomas and hyperplasia that could be prevented by continuous tetracycline administration. We have adopted this transgenic system to the regulated expression of HCV proteins in the liver using this model system. Analysis of Alb-tTA/TRE-HCV double transgenic mice revealed that the HCV RNA and protein expression in the liver could be completely suppressed by tetracycline administration, and induced in a reversible fashion by tetracycline withdrawal. Mice with constitutive expression of HCV had no evidence of hepatic pathology until 11 months of age, when steatosis developed. In contrast, mice with tetracycline-mediated suppression and then withdrawal at 2 months of age developed hepatic inflammation with ALT elevation by 4 months of age. Hepatic steatosis also became evident at 5 months of age, which occurred much earlier than the mice with constitutive HCV expression. Neither hepatic inflammation nor steatosis was observed in mice that had been continuously subjected to tetracycline treatment. Several cytokines including IL-6, -8, -9, -10 and IFN-g are produced in the liver concurrently with the induced expression of HCV, suggesting that these cytokines may play a role in the pathogenesis of HCV infection in vivo. Finally, induction of HCV expression after birth by tetracycline withdrawal was associated with the appearance of HCV-specific immune response, suggesting an immune-mediated injury in this model. With a tightly regulatable HCV expression, this model resembles those events observed in natural infection and offers a valuable tool to study the pathogenesis of hepatitis C. We have also initiated studies in chimpanzees, which are the only infectious animal model for hepatitis C. We have constructed a molecular infectious clone of HCV genotype 1b and studied the detailed virologic, serologic, and immunologic parameters of four chimpanzees inoculated with this monotypic inoculum. Two of the chimpanzees cleared the infection and two became persistently infected. Only weak and transient T helper responses were detected during the acute phase in all four chimpanzees. To further explore the functional differences among these chimpanzees, cDNA microarray experiments are being conducted to evaluate the global gene expression profiles of these chimpanzees. These animal models are valuable not only to address issues of immunopathogenesis and cytopathic potential of HCV gene products but also to study HCV replication in vivo.